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FI magazine, Vol. 1, issue 10' 1996
Dick Olsher
 

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Every hi-fi component has a mission in life, a unique place in the audio chain. Line-level preamps, a.k.a. line stages, typically take on three specific tasks: source selection, volume/balance control, and voltage amplification. Since line level sources (e.g., CD players, cassette decks, and tuners) typically range from 0.1 to 1 volt in output signal, the preamp at most needs to provide an additional 20 dB of gain to drive power amps of average sensitivity to full output. Sounds easy, no? So why is there such a shortage of great sounding line stages on the market?

This rather depressing observation emerged after a several-year-long quest for the ultimate statement in this genre. No matter what the asking price, I was never completely happy. There was always a fly in the ointment, some performance aspect that barred a perfect "10" rating. However, a pattern became clear: the common denominator shared by the best sounding units was power supply sophistication. I could almost predict the ultimate ranking of a particular model solely on the basis of the effort expended on the power supply.

Hah? I can see the puzzled looks. What's the power supply got to do with it? Sound quality is known to be affected by the choice of gain devices, signal path caps and resistors, internal wiring, connectors, and circuit boards. The unglamorous power supply is often overlooked because it is considered to be out of the signal path. Consequently, most preamps feature a bare bones, inexpensive power supply. The truth is that nothing is out of the signal path, especially the power supply. It is literally the engine that drives the amplifier. It fixes the preamp's operating point and provides the idle currents for the gain devices. It's a major route of entry for RF hash. Inadequate rectification, filtering, and reservoir capacity can all affect signal purity. And unless critical voltages are regulated, the preamp's operating point will drift in concert with the line voltage. Yet, it is rare to find a preamp with any voltage regulation.

Leave it to LAMM's Vladimir Shushurin to nail down a definitive high-voltage series regulator. Not only that, but in an otherwise solid-state design, the L1's regulator is all-tube. The regulator is built around a 6C19Pi, a Russian tube that is the younger brother of the 6C33C. A 12AX7 triode and a 5651 glow-discharge diode are also used in the regulator.

The overall topology of the L1 is quite simple: a single gain stage followed by a buffer. The buffer uses four high-current, high-frequency MOS-FETs, and is similar to that used in the ML1 power amp. The resultant output impedance is only 130 Ohms, in contrast with the 1,000 Ohms or so common to vacuum-tube preamps. The real-world benefit of this is the L1's ability to drive long cables, exotic interconnects, and any power amp known to man. The active circuit blocks are MOSFETs, operated in class A with zero feedback. The intrinsic linearity of the gain blocks minimizes distortion products even with no feedback. All MOSFETs are hand selected. Supertex DMOSFETs were chosen for the input stage because they can operate at 300 volts, are low-noise and exceptionally linear.

A total of five inputs and two tape loops with dubbing capability are provided. A nice feature is the DIRECT input, which bypasses the MONITOR, RECORD, and MODE switches. Instead of a standard balance control, the L1 uses a pair of attenuators to adjust channel gain. The advantage is that the overall gain of the preamp can be matched to meet the needs of the power amp. All the inputs are unbalanced. However, both balanced and unbalanced outputs are available.

The most surprising thing about the L1 was its mastery over the low end. Perhaps an overexposure to vacuum tube designs has diminished my expectations as far as bass performance goes. Whatever the case, I wasn't prepared for the L1's kick-ass attitude. I'm referring specifically to the bass range from 40Hz to about 300Hz. There's nothing interesting musically for me in the lowest octave, below 40Hz. However, the mid and upper bass is where all the bass action is for most instruments, including piano, double bass, and cello. The fundamental together with the first two harmonics define bass weight. (Pitch perception depends on the periodicity of an instrument's harmonic envelope; the fundamental need not even be present. For example, a signal made up of a harmonic series with partials at 400, 600, 800 and 1,000Hz, will be perceived with a pitch of 200Hz--the frequency of the missing fundamental. Of course, timbre will be affected when the fundamental is missing or reduced in amplitude.) While the L1, I'm sure, doesn't know about psychoacoustics, it did everything right in the bass. It sounded totally cohesive so that the body of each instrument ebbed and flowed with absolute precision and clarity. Bass lines were easy to resolve, as the leading edge of each note remained distinct. Contrast this with the noxious practice of a horde of competing line stages which smear the attack portion of a transient to the point of blending bass lines.

The L1 put the kick back into kick drum. It seemed to focus the bass energy in time so that a drum's punch and visceral impact were never more realistic. Its sense of timing was superb. The rhythmic drive of a musical ensemble in full tride was mesmerizing. And it did all this legato style, with smooth mids, and well-behaved treble transients.

Despite all those MOSFETs in the signal path, the sound was far removed from that of pure solid-state designs. Oh, there are numerous smooth-talking transistor beauties out there. But they all lack one essential characteristic: they fail to retrieve the music's full emotional content. Doing that well is contingent on the ability of a device to retrieve the micromodulations coded into the audio signal. Both low-level volume and frequency modulations allow us to perceive feelings and moods. These subtle cues work by enabling us to sort and fuse harmonics with similar modulations from the jumbled mess of clues present in the auditory stream. The L1's expressive palette isn't perfect, but it blows all-transistor designs out of the water. All of which brings me to Madama Butterfly.

Although I enjoy opera, my forays to the Santa Fe Opera have been pretty limited. For one thing, there's the issue of comfort. Since the Opera is performed in the open, rain and especially cool summer nights are a deterrent. Unless you're properly dressed, hypothermia is a real possibility. And that's no exaggeration; my life almost ended several years ago at the Opera. But I actually prefer opera in the comforts of my listening room for a more fundamental reason--the fat-lady syndrome. I find the visual incongruity between the main characters and their flesh-and-blood embodiments distracting. Leopold Stokowski put it best in an address delivered extemporaneously before the New York meeting of the Acoustical Society on May 2nd, 1932: "You go to Tannhaüser, and the idea of Tannhaüser is that Venus, the most beautiful woman who ever existed should tempt Tannhaüser from the narrow path of virtue. But unfortunately Venus was chosen because she had a marvelous larynx and unfortunately sometimes she weighed too many pounds and unfortunately---but I leave that to your imagination. And so the whole evening, the whole point and meaning of that drama is changed and spoiled because you couldn't imagine anybody (laughter) well...It is such a pleasure to talk to physicists. They understand everything!"

So I trot out Madama Butterfly and prepare to let my imagination fill in the missing faces. And it is no ordinary performance, with Mirella Freni and Jose Carreras, and Giuseppe Sinopoli conducting the Philharmonia Orchestra [DG 423567-2]. (Thank you brother Jim Van Sant for introducing me to this recording.) And I'm immediately hooked. This outing, love and death within the confines of a soundstage seemed believable. The power and drama, the drive and dynamics of the original performance vanquished the constrains of the equipment. Butterfly's last moments touched me. And that's the most we can ever hope for from an audio system.

The L1's speed and control were also evident in the superb reproduction of sibilants and in its resolution of massed voices and instruments. Numerous experiments with different signals and in a variety of tasks has established the resolution of the auditory system at about 2 milliseconds. (Most researchers look to measure the threshold for discrimination for a temporal gap in a particular signal.) Listening to an ensemble or massed voices performing in unison presents a musically meaningful setting for assessing temporal resolution. Timing asynchronies between musicians of up to about 30 milliseconds are normally tolerable, before it becomes apparent that somebody is out-of-sync. However, even timing differences of several milliseconds should be readily resolvable on a good system. Bach's "Komm, Jesu, komm," on the Grundig demo CD [Fine Arts, GL3322] is excellent for this purpose. With the L1 I was able to resolve almost every individual soprano and tenor in the chorus.

...On balance, I find the L1 to be easy to live with. Its suave mids, speed, transient control, and stupendous bass drive satisfy most of my sonic priorities. It eclipses the Audio Research LS-5, and a host of expensive solid-state units. A mandatory audition at its asking price.

 

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